This invention relates to wireless personal portable communications systems and more particularly, to a portable handset unit for use in such a system that is compatible for operation in two different frequency bands that by their nature require separate modes of operation.
The New York Times, on page 1 of the Business Day section, on Monday, Dec. 5, 1994, reported an auction to take place that day for "99 licenses to use the radio spectrum for `personal communication services,` a new family of wireless telephone and data services. . . . [T]he total of the winning bids is expected to be somewhere between $3 billion and $15 billion." The article quotes Reed E. Hundt, chairman of the Federal Communications Commission (FCC) as saying: "This auction represents the largest one-time launching of a start-up industry in American's peace-time history." For each geographical area, licenses will be granted to allow the provision of personal communications services in lower and upper frequency bands of 1850-1910 MHz and 1930-1990 MHz. Licenses to service providers will be for either a pair of 15 MHz or 5 MHz wide subbands, the first subband in each pair being in the lower frequency band and the second subband of the pair being in the upper frequency band, separated from the first by 80 MHz.
The FCC allocated such frequencies in what is known as the Emerging Technologies (ET) Band in the FCC Second Report and Order, "Amendment of the Commission's Rules to Establish New Personal Communications Services", Oct. 22, 1993, GEN Docket No. 90-314, and in the FCC Memorandum Opinion and Order, "Amendment of the Commission's Rules to Establish New Personal Communications Services", Jun. 13, 1994, GEN Docket No. 90-314. The Second Report and Order and the June 1994 Opinion and Order defined the aforenoted licensed bands which were set aside for isochronous operation, intended primarily for circuit oriented voice and data applications. They also defined an unlicensed spectrum between 1910 MHz and 1930 MHz of which the band from 1910 to 1920 MHz is set aside for asynchronous operation, intended primarily for packet data, and the band for 1920 to 1930 MHz is for isochronous operation, again intended for circuits.
Within the licensed band, each licensed service provider need only comply with broad technical rules, employing any one of several candidate technologies in the provision of wireless personal communications services, such as voice and data. Such personal communications services are envisioned to supplement, not replace, current cellular mobile radio systems, which are generally designed for analog voice transmission in a lower frequency band of 824-889 MHz. PACS (personal access communications systems) has emerged as a leading candidate among low-tier systems proposals in the Joint Technical Committee (JTC) of ANSI T1 and T1A. Unlike current high-tier cellular systems, which characteristics include high power (&gt;1 watt), large cell size (radius.apprxeq.2 miles), and high mobility (up to 100 mph), the characteristics of PACS include low power (&lt;1 watt), small cell size (radius.apprxeq.200 meters), and low mobility (&lt;40 mph). Advantageously, PACS in the licensed spectrum can provide higher speech quality and lower signal delay than vehicular cellular systems, primarily for pedestrian public access in outdoor and some indoor venues such as shopping malls and airports. For such pedestrian usage, the portable transceivers used by each subscriber must be small, lightweight, and relatively inexpensive. In addition, power consumption must be low to provide long-life between recharging.
In view of the split configuration of the licensed subbands in the licensed spectrum, the PACS proposal uses frequency-division duplexing (FDD) techniques which are best suited to separate uplink transmissions (from a portable handset to a fixed port) from downlink transmissions (from a fixed port to a portable handset). By operating the multiplexed radio links on a time-division multiplexed/time-division multiple access (TDM/TDMA) basis, a number of separate fully duplex demand-assigned digital channels can be provided to allow a number of portables to simultaneously access a single port on a multiplexed basis. Specifically, in the PACS proposed arrangement each port transmits time-division multiplexed (TDM) bit streams on a predetermined downlink carrier frequency, with, in turn, each portable that accesses that port responding by transmitting a TDMA burst on a corresponding uplink carrier frequency (equal to the uplink frequency less the 80 MHz separation between uplink and downlink subbands).
The PACS proposal uses spectrally-shaped quadrature phase shift keying (QPSK) at 192 kbaud (384 kbits/s). Each digital radio frame is 2.5 msec long and comprises 8-312.5 .mu.sec bursts per frame, numbered 0 to 7. This short frame length was chosen for its small round-trip delay, which for speech, eliminates the need for echo control, and for data, allows for the rapid acknowledgement of data packet reception for high data throughput. It also allows channel access and handoff by a portable of an active call to another port to proceed at a rapid pace because measurements can be made faster. The channel data rate was chosen for its tolerance to delay spread.
In a burst window of 120 bits, 80 bits (10 octets) are allocated to the fast channel (FC). The FC provides a raw data rate of 32 kbps, suitable for reasonable quality speech coder. Inter-carrier spacing of 300 KHz is used for both uplink and downlink channels. The power used by the transmitter in the portable will range between 5-10 milliwatts or less on average and provide a range of several hundred to a thousand feet. As such, the resulting low radiated power poses essentially no biological radiation hazard to any user. In addition, the port antenna can be relatively small and suitable for mounting on a utility or light pole. With this transmission range, a port can simultaneously serve typically 20-30 separate locally situated portables. The same TDM downlink channels are reused at ports that are spaced sufficiently far apart to reduce co-channel interference to an acceptably low level but yet conserve valuable spectrum. To provide access to the wireline telephone network, each port is interfaced, typically through a conventional fixed distribution facility, over either a copper or fiber connection to a switching machine at a local central office.
For high performance, a portable in PACS implements microscopic diversity in both receive and transmit operations, which means determining which of multiple transmission paths gives the best signal performance. It has been shown that a 10 dB performance improvement in signal-to-noise ratio can be obtained by employing diversity techniques. Thus, each portable unit includes multiple antennas with the portable's electronics intelligently selecting signals from these antennas to combat the effects of signal fading. Such antenna selection is performed prior to the reception of each burst. In an FDD mode, the selection procedure must be performed at the portable rather than at the port since determination of the preferred antenna must be made based on measurements at the downlink frequencies which are available only at the portable in order to determine over which antenna a "best" quality signal is received. Accordingly, during the period of each downlink frame, between bursts from the port to that portable, time must be allocated for the portable to make measurements of the signals received on both antennas, and the receiver is then switched to the "better" input to receive its intended burst from the port. This is called preselection diversity.
Implicit with the requirement for high performance in PACS is the capability of a portable during an active call to make continuous signal measurements of other radio ports in the system to support mobile-controlled handoffs to another port. Thus, as subscribers move their portable from an area proximate to one port to another closer port, or as changing conditions result in a deteriorated signal quality with the original port and that a port with a better signal quality has an available channel, the switching machine would be suitably programmed to controllably and automatically handoff calls from the original port to the closer or "better" port at the direction and request of the portable. Accordingly, during a period of each downlink frame, between bursts from the port to that portable, time must also be allocated for the frequency synthesizer in the portable to tune to the downlink carrier frequency of a port other than the one it is presently communicating with and make measurements on both antennas at that other port's downlink frequency. After such measurements, the receiver must then retune to its original downlink frequency to receive the next burst from the original port with which it is communicating.
Various aspects of TDMA for PACS have been treated in prior art patents such as U.S. Pat. No. 4,849,991, entitled "Method and Circuitry for Determining Symbol Timing for Time Division Multiple Access Radio Systems," issued Jul. 18, 1989 to H. W. Arnold and N. R. Sollenberger, two of the co-inventors herein; U.S. Pat. No. 4,937,841, entitled "Method and Circuitry for Carrier Recovery for Time Division Multiple Access Radio Systems," issued Jun. 26, 1990 to J. C. Chuang and N. R. Sollenberger; U.S. Pat. No. 4,941,155, entitled "Method and Circuitry for Symbol Timing and Frequency Offset Estimation in Time Division Multiple Access Radio Systems," issued Jul. 10, 1990 also to J. C. Chuang and N. R. Sollenberger; U.S. Pat. No. 5,084,891, entitled "A Technique for Jointly Performing Bit Synchronization and Error Detection in a TDM/TDMA System," issued Jan. 28, 1992 to S. Ariyavisitakul, L. F. Chang (also an inventor herein) and N. R. Sollenberger; U.S. Pat. No. 5,155,742, entitled "Time Dispersion Equalizer Receiver with a Time-Reversal Structure for TDMA Portable Radio Systems", issued Oct. 13, 1992 to S. Ariyavisitakul and H. W. Arnold; U.S. Pat. No. 5,177,769, entitled Digital Circuits for Generating Signal Sequences for Linear TDMA Systems", issued Jan. 5, 1993 to H. W. Arnold and N. R. Sollenberger; U.S. Pat. No. 5,212,831, entitled "Method and Apparatus for Autonomous Adaptive Frequency Assignment in TDMA Portable Radio Systems", issued May 18, 1993 to J. C. Chuang and N. R. Sollenberger; U.S. Pat. No. 5,222,101 entitled "Phase Equalizer for TDMA Portable Radio Systems", issued Jun. 22, 1993 to S. Ariyavisitakul and H. W. Arnold; U.S. Pat. No. 5,226,045 entitled "Method and Apparatus for Autonomous Selective Routing During Radio Access in TDMA Portable Radio Systems", issued Jul. 6, 1993 to J. C. Chuang; U.S. Pat. No. 5,333,175, entitled "Method and Apparatus for Dynamic Power Control in TDMA Portable Radio Systems", issued. Jul. 26, 1994 to S. Ariyavisitakul, J. C. Chuang and N. R. Sollenberger; U.S. Pat. No. 5,363,375, entitled "Method and Apparatus for Synchronizing Timing Among Radio Ports in Wireless Communications Systems Using Hierarchical Scheme", issued Nov. 8, 1994 to J. C. Chuang and N. R. Sollenberger; and U.S. Pat. No. 5,363,376, entitled "Method and Apparatus for Synchronizing Timing Among Radio Ports in Wireless Communications Systems", also issued Nov. 8, 1994 to J. C. Chuang and N. R. Sollenberger.
As aforenoted, the FCC it its Second Report and Order of October 1993 and its Memorandum Opinion and Order of June 1994 also created a 10 MHz wide band from 1920 MHz to 1930 MHz for isochronous or circuit based unlicensed wireless access. It is envisioned that PACS in the unlicensed band (PACS-UB) could be used for wireless Centrex, or PBX applications, as well as private residential applications. For such residential applications, PAC-UB could replace present cordless telephone sets, but with better range and voice quality than is provided in the frequency band of presently available cordless units. A primary requirement for any such PACS-UB system is their compliance to new FCC Rules in Part 15, Subpart D, which define an "etiquette" by which unlike systems can make common use of the allocated spectrum. Such Rules have two principal goals: to enable emerging technologies to be deployed in a coordinated fashion with existing microwave systems which currently use that spectrum (until the spectrum is cleared); and to enable unlike systems to reasonably share the same spectrum. Among the various constraints and requirements of the Rules is a maximum transmit power level, which limits PACS-UB to primarily indoor use.
The Rules do not require a conforming system to operate in a time-division duplex (TDD) mode in which separate time-slots at the same carrier frequency are used for uplink and downlink transmissions. However, only TDD systems can feasibly operate in the unlicensed band, because sufficient frequency duplex separation is not available to isolate the transmissions in each direction. Although the Rules do not require that a portable handset be compatible for operation in both the licensed spectrum and the unlicensed spectrum, interoperability of portable handsets between the licensed and unlicensed bands is encouraged to enable a subscriber to the licensed band to use his same handset within a local environment in the unlicensed band. Such a subscriber, who will pay service providers for "air time" for his usage in the licensed band, will not have such charges associated with his access of the unlicensed band when accessing the telephone network, as cordless telephone users do not now. Thus, unlike current cellular telephone subscribers who may use their cellular telephone from their home to make a local call and are charged air-time for the such a call, PACS subscribers may use their handset within their home, if equipped with an unlicensed band terminal, without being charged by their licensed band service provider. Also, use of the unlicensed band has much more attractability from a cost basis if a portable handset can also operate in the licensed band. Interoperability of portable handsets between the two bands will also encourage manufacturers to manufacture equipment for use in the unlicensed band, which without such interoperability they may not be so stimulated to do.
In order to be compatible in both the licensed and unlicensed bands, a portable handset must be capable of operating in both and FDD and TDD modes. An overall design objective for both modes of operation is, however, low cost, low complexity, and high performance. It is thus necessary to use a design incorporating a major reuse of hardware for both modes of operation, but which still provides required high performance.
A critical component in a hardware design of a portable handset for use in either the FDD mode or TDD mode is a frequency synthesizer. In the TDD mode, during an active call the same frequency is used on the uplink and downlink channels. Within each frame period, however, measurements must be made on other channels for purposes of determining whether handoff to another port should be made, thus requiring retuning of a synthesizer to at least one other another channel in a frame. In the FDD mode during an active call, and within each frame period, separate uplink carrier and downlink carrier frequencies must be generated for transmitting and receiving the bursts to and from the port with which the portable is communicating. In addition, within each frame, retuning to at least an additional downlink carrier frequency for handoff procedures is also necessary. Thus multiple carrier frequencies are required within the short time period of a frame, requiring either a fast-switching synthesizer or multiple synthesizers, either of which would add significant complexity and cost to the portable. These solutions negate the design objectives. A radio design that could accommodate a less expensive slow-switching synthesizer is therefore desirable.
In addition to the synthesizer tuning that must be performed that can occupy a significant portion of the frame period, time must be allocated in each frame period for the aforementioned diversity measurements for desired high performance. By using a frame structure having a longer duration, the required tuning functions and the diversity measurements could easily be accommodated. A short frame duration has many advantages, however. These include: the absence of any echo control requirement; a rapid speech recovery on errors; the support of higher user velocities during operation; fast call setup and handoffs; and for data transmission, low-delay error retransmission. Thus, concomitant with a longer frame duration would be poor quality.
An object of the present invention is to provide a low-complexity, high-performance portable handset design for PACS that is compatible for operation in both the licensed and unlicensed portions of the Emerging Technologies frequency band.